Alkenes Flashcards
How do alkenes react?
Addition. The double bond is unsaturated
Catalytic Hydrogenation
Uses a metal catalyst like PtO2, Pd, which H and the Alkene both bond to. The reaction is stereospecific – syn addition of hydrogens from either the bottom or the top.
Electrophilic Addition
The hydrogen of a strong acid can attack one of the carbons in the double bond, reverse of E1. This creates a carbocation, which a nucleophile may attack. Watch out for rearrangements, and for solvents. If the solvent is an ok nucleophile, there’s a chance it will act like one (because it’s more abundant that the base which the H detached from). The nucleophile may be trapped from either direction – syn or anti addition. If non-symmetric alkene, the H goes for the less substituted carbon (Markovnikov’s rule)
Hydrohalogenation
H-X reacts with alkene through electrophilic addition . Can be syn or anti, and can include carbocation rearrangement. Also watch for solvent – it might lead to the major product.
Markovnikov Hydration
In the presence of a strong acid (H2SO4) with excess water in low temperatures, alkenes reverse the dehydration process and become alcohols. The equilibrium between the two is controlled by temperature – at high temperature, and with not excess water, it goes the other way. Carbocations happen.
Equilibration of alkene products
When alcohols undergo E1, the most stable products become the most common. The less stable alkenes go back to being alcohols, and then go forward again. The stable ones don’t go back, so they eventually become more numerous.
Halogenation
Works best for chlorine and bromine. The alkene reacts with a Br from a Br-Br molecules, stealing it and creating a cyclic bromonium ion. The bromine in the ring has a positive charge here. Then, the left behind bromine (which is negative) attacks the more substituted carbon, opening the ring, and resulting in anti stereochemistry. Watch for other nucleophiles which could intercept the bromonium, such as water, CN, RS
Oxymercuration - Demurcuration
A Markovnikov hydration that avoids cations (no rearrangement). Uses H2(O2CCH3)2. One of the mercuric acetates dissociates, leaving H2(O2CCH3), which is very electrophilic. It attacks the double bond, becoming a cyclic ion. Water acts as a nucleophile and opens the ring (attacks at more substituted C). This part gives an anti ion with the mercuric acetate thing hanging off one carbon. It is reduced by NaBH4, leaving an alcohol.
Hydroboration - Oxidation
Allows anti-Markovnikov hydration (!). Uses BH3, which displays syn addition of an H and a BH2, in which the B goes to the less hindered end of the double bond. Oxidation of resulting alkylboranes with H2O2 and NaOH creates anti-Markovnikov alcohols.
Carbene
A :CR2 group, in which the central carbon has a sextet. Extremely reactive!
Carbene origins
Diazomethane (CH3-NN) can be reacted with light, heat, or copper to form methylene
Chloroform (CH)Cl3 can make dichlorocarbene
Simmons - Smith reagents are diiodomethanes treated with zinc to generate ICH2ZnI, which is a carbeniod (it acts like a carbene, but isn’t exactly)
Cyclopropane synthesis
Carbene reacts with an alkene, forming a cyclopropane. Syn addition (kind of, but only one thing is being added).
Oxacyclopropane (Epoxide) synthesis
The electrophilic oxygen in the OH group of a peroxycarboxylic acid (RC=OOOH) adds to a double bond to form an oxacyclopropane, with a carboxylic acid as the other product. Commonly referred to as epoxidation. Peracetic acid, MCPBA or MMPP are common reagents for this. The addition of oxygen is syn, with the stereochemistry of the starting alkene being retained. Concerted mechanism – everything happens at once. If there are two double bonds, it attacks the more substituted one first!
Epoxide application
Can be used to create syn addition of two nucleophiles. first, one nucleophile attacks, creating the alcohol on the opposite carbon, and inversion at the carbon of Nu attack. then, alcohol can be protonated and hit with a second attack, upon which it leaves, and the second nucleophile attaches to its carbon, syn to the other Nu.
Anti-dihydroxilation of alkenes
From an oxacyclopropane, two ways. Either with H+ and H2O, or with OH- and H2O. Either way, an OH group gets attached as a nucleophile, on the other carbon and anti to the oxygen which leaves the ring. Ta da
